1. Field of the Invention
The present invention relates to image processing methods and apparatuses, and more particularly, to a method for processing digital image signals.
2. Description of the Related Art
Thin display devices, liquid crystal displays (LCDs), plasma displays (PDPs), and field emission displays (FEDs) have attracted attention.
The LCDs, PDPs, and FEDs are fixed-pixel matrix-driven display devices, which can be driven by digital image signals. The number of grayscale levels of the above-described display devices is represented by the number of bits of a video signal corresponding to each pixel.
Techniques for displaying images so that they can be visually aesthetic to the human eye by performing signal processing on image signals are being considered. Such techniques include edge enhancement processing for enhancing edge portions and high-frequency components of images so as to apparently increase the resolution of the images.
FIG. 8 illustrates the configuration of an edge enhancer 800 for performing edge enhancement processing on image signals.
An image signal input from an input terminal 801 is output to a high-pass filter 803 and also to an adder 807.
The high-pass filter 803 extracts high-frequency components of the input image signal and outputs the resulting image signal to a multiplier 805.
Under the control of a controller 809, the multiplier 805 multiplies an enhancement coefficient, indicating the level of enhancement of the high-frequency components of the image, by the high-frequency components, and outputs the resulting signal to the adder 807. By controlling the enhancement coefficient, the level of enhancement of the high-frequency components of the image can be adjusted.
As the bit precision of the high-frequency components which are output from the multiplier 805, an 8-bit image signal input from the input terminal 801 can be increased to a 12-bit image signal by the high-pass filter 803 and by the multiplier 805.
Then, the adder 807 adds the original 8-bit image signal and the 12-bit high-frequency components output from the multiplier 805 and outputs the resulting 12-bit image signal having enhanced high-frequency components to a rounding unit 811.
By outputting an enhancement coefficient having a negative sign from the controller 809, the image can be made smoother instead of enhancing the edges.
To convert the 12-bit image signal into an 8-bit image signal, the rounding unit 811 truncates the lower four bits of the 12-bit image signal by a rounding operation, and outputs the resulting 8-bit image signal to an output terminal 813.
FIG. 9 illustrates the configuration of an image processing apparatus for performing resolution conversion on an image signal before performing edge enhancement.
An image signal input from an input terminal 901 is supplied to a resolution converter 903.
The resolution converter 903 converts the resolution of the input signal to the resolution of a display device (not shown) under the control of a controller 909.
If, for example, the horizontal resolution and the vertical resolution of the display device are 1280 pixels and 720 pixels, respectively, and if the number of horizontal pixels and the number of vertical pixels of an input image signal are 720 and 480, respectively, the horizontal resolution and the vertical resolution are scaled up by 16/9 and 3/2, respectively.
If, for example, the horizontal resolution and the vertical resolution of the display device are 1280 pixels and 720 pixels, respectively, and if the number of horizontal pixels and the number of vertical pixels of an input image signal are 1920 and 1080, respectively, the horizontal resolution and the vertical resolution are scaled down by ⅔ and ⅔, respectively.
If the input pixel signal has 8 bits, the resolution converter 903 expands the pixel signal into a 10-bit pixel signal to maintain the precision of the converted pixel signal, and outputs the 10-bit pixel signal to an edge enhancer 905.
Under the control of the controller 909, the edge enhancer 905 enhances high-frequency components of the image and outputs the resulting image signal to a rounding unit 907.
If the input pixel signal has 10 bits, the edge enhancer 905 expands the pixel signal into a 12-bit pixel signal to maintain the precision of the image signal with enhanced edges, and outputs the 12-bit pixel signal to the rounding unit 907.
To reduce the 12-bit pixel signal into an 8-bit pixel signal, the rounding unit 907 truncates the lower four bits of the pixel signal, and outputs the resulting 8-bit data to an output terminal 911.
Meanwhile, the controller 909 controls the resolution conversion (scaling) ratio in the resolution converter 903 and also controls the level of edge enhancement in the edge enhancer 905.
In printers, halftone processing using dithering processing has been performed as a binarizing method. In printers, such as that disclosed in Japanese Patent Laid-Open No. 2000-134471, images are divided into, for example, a character portion and a photograph portion, and different binarizing methods are used for these portions.
Japanese Patent Laid-Open No. 2003-69830 discloses an image processing method for performing resolution conversion and dithering processing.
When edge enhancement is conducted after performing resolution conversion and when the resulting image is then rounded, pseudo contours may easily occur depending on the type of image.
When enlargement processing is performed, the correlation of adjacent pixels becomes high, and pseudo contours easily occur, which is visually noticeable.
To prevent the occurrence of pseudo contours, dithering processing can always be performed instead of the rounding operation. In this case, however, dithering processing does not produce a noticeable effect on images subjected to resolution reduction processing or edge enhancement, since pseudo contours do not easily occur because of the low correlation between adjacent pixels of such images. Conversely, dithering processing easily produces an adverse influence, for example, noise having a fixed pattern, which is noticeable.